And here is a trick.
You may apply a general patch for all Active Admin resources to pass your model's error message to the user as a ResourceController's callback. It's the check_model_errors method below. And this method has to be registered as a callback during execution of each resource's ActiveAdmin.register method call (see the patched run_registration_block).
You may simply paste the code below to a new file (of any name) to the config/initializers folder of your app (or any other folder which is initialized upon app startup). I put it as config/initializers/active_admin_patches.rb.